1. Field of the Invention
The present invention relates to an electromagnetic induction device of the type in which coils of respective phases are cooled by a flow of a cooling medium composed of an insulating gas such as SF.sub.6 gas. More particularly, the present invention is concerned with an electromagnetic induction device improved to equalize the flow rates of the cooling gas through the coils of all phases.
2. Description of the Related Art
FIG. 3 is a schematic sectional view of a 3-phase electromagnetic induction device as an example of conventional electromagnetic induction devices. Referring to this figure, a tank 1 accommodates coils 2A, 2B and 2C of A, B and C phases which form a major part of the electromagnetic induction device and which are illustrated schematically. These coils 2A, 2B and 2C will also be collectively referred to as coils 2. One end of a lower coolant pipe 3 is connected to and opens into a lower portion of the tank 1 so as to introduce a flow of a coolant to a space under the electromagnetic induction device. Upper coolant pipes 4, each connected at one end to a cooler (not shown), are connected at the other end to a top wall of the tank 1. A coolant duct 8 is defined between the bottom wall of the tank 1 and a partition plate 5. The partition plate 5 has openings which provides coolant inlets 5A, 5B and 5C for introducing the coolant to the coils 2A, 2B and 2C of the respective phases. In this known electromagnetic induction device, a flow of a coolant produced by a blower is supplied into the coolant duct 8 through the lower coolant pipe 3 and is then introduced, as indicated by arrows, into the coils 2A, 2B and 2C of the respective phases through the coolant inlets 5A, 5B and 5C formed in the partition plate 5, thereby to cool these coils 2A, 2B and 2C. The coolant after cooling the coils 2A, 2B and 2C is then introduced into the cooler through the upper coolant pipes 4. Thus, the flow of the coolant is forced by a blower into the coolant duct 8, and the flow of the coolant is distributed to the coils 2A, 2B and 2C. In the distributed coolant flow from the coolant duck 8 to respective coils 2A, 2B and 2C, a deceleration caused by a flow distribution of the coolant acts as a pressure buildup in the coolant, and a frictional pipe resistance acts as a pressure drop in the coolant. As a consequence, the coolant is distributed to the coils 2 unevenly such that the flow rate is smallest in the coil 2A of the phase A nearest to the lower coolant pipe 3 and greatest in the coil 2C of the phase C remotest from the lower coolant pipe 3.
The uneven distribution of the coolant to the coils 2A, 2B and 2C causes a difference in the rate of conveyance of heat from these coils to the cooler. Consequently, the coil 2A of the phase A in which the coolant flow rate is smallest may exhibit a temperature rise to a level exceeding the rated temperature. This promotes deterioration of the insulating material forming the coils 2 to shorten the life of the electromagnetic induction device.